They asked when they would go to Beijing.求原因,

They asked when they would go to Beijing.求原因,
They asked when they would go to Beijing.
为什么要用would?
我对这个句子充满了混乱.求翻译,求用法,求原因..
老师讲的很..囧..老师说”因为是asked所以不用will就用would“.实习老师你情何以堪我听不懂啊!